State clearly what lengths mean for stream and datagram data moved

[LPardue]

Addresses part of #375

[rmarx]

As discussed during editorial meeting:
I'm not the biggest fan of current text since it isn't clear for all the possible uses (e.g., transport to application strips QUIC level stuff but keeps H3 frame headers, while application to user also strips those out etc.). Maybe best to word it in terms of "only the payload is moved to the higher layer" or something like that?